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      Volume 68, Issue 3

      March 2007,   pages  377-534

    • Effects of barrier fluctuation on the tunneling dynamics in the presence of classical chaos in a mixed quantum-classical system

      Aparna Saha Bidhan Chandra Bag Pranab Sarkar

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      We present a numerical investigation of the tunneling dynamics of a particle moving in a bistable potential with fluctuating barrier which is coupled to a non-integrable classical system and study the interplay between classical chaos and barrier fluctuation in the tunneling dynamics. We found that the coupling of the quantum system with the classical subsystem decreases the tunneling rate irrespective of whether the classical subsystem is regular or chaotic and also irrespective of the fact that whether the barrier fluctuates or not. Presence of classical chaos always enhances the tunneling rate constant. The effect of barrier fluctuation on the tunneling rate in a mixed quantum-classical system is to suppress the tunneling rate. In contrast to the case of regular subsystem, the suppression arising due to barrier fluctuation is more visible when the subsystem is chaotic.

    • Single photon and nonlocality

      Aurelien Drezet

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      In a paper by Home and Agarwal [1], it is claimed that quantum nonlocality can be revealed in a simple interferometry experiment using only single particles. A critical analysis of the concept of hidden variable used by the authors of [1] shows that the reasoning is not correct.

    • On relativistic models of strange stars

      Ramesh Tikekar Kanti Jotania

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      The superdense stars with mass-to-size ratio exceeding 0.3 are expected to be made of strange matter. Assuming that the 3-space of the interior space-time of a strange star is that of a three-paraboloid immersed in a four-dimensional Euclidean space, we obtain a two-parameter family of their physically viable relativistic models. This ansatz determines density distribution of the interior self-gravitating matter up to one unknown parameter. The Einstein's field equations determine the fluid pressure and the remaining geometrical variables. The information about mass-to-size ratio together with the conventional boundary conditions lead to the determination of total mass, radius and other parameters of the stellar configuration.

    • A study of daily variation in cosmic ray intensity during high/low amplitude days

      Rajesh K Mishra Rekha Agarwal Mishra

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      A detailed study has been conducted on the long-term changes in the diurnal, semi-diurnal and tri-diurnal anisotropies of cosmic rays in terms of the high/low amplitude anisotropic wave train events (HAE/LAE) during the period 1981-94 using the neutron monitor data from Deep River Neutron Monitoring Station. In all, 38 HAE and 28 LAE cases have been studied. An inter-comparison of the first three harmonics during these events has been made so as to understand the basic reason for the occurrence of these types of events. It has been observed that the phase of diurnal anisotropy shifts towards earlier hours for HAEs and it shifts towards earlier hour as compared to 18-h direction for LAEs. For semi-diurnal anisotropy, phase remains statistically the same for both HAE and LAE. In the case of tri-diurnal anisotropy, phase is evenly distributed for both types of events. The interplanetary magnetic field (IMF) and solar wind plasma (SWP) parameters during these events are also investigated. It has also been observed that HAE/LAEs are weakly dependent on high-speed solar wind velocity. The two types of solar wind streams (corotating streams and flare-generated streams) produce signi¯cant deviations in cosmic ray intensity during HAE/LAE.

    • Beta decay rates of nuclei with $65 < A < 75$ for pre-supernova and supernova evolution

      Debasish Majumdar Kamales Kar

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      The half-lives are calculated for the $\beta^{-}$ decay process for nuclei in the mass range $\sim 65-75$ relevant for the core of a massive star at the late burning stage of stellar evolution and the collapse that leads to supernova explosion. These half-lives and rates are calculated by expressing the $\beta^{-}$ Gamow-Teller decay strengths in terms of smoothed bivariate strength densities. These strength densities are constructed in the framework of spectral averaging theory for two-body nuclear Hamiltonian in a large nuclear shell model space. The method has a natural extension to electron captures as well as weak interaction rates for 𝑟 and $rp$-processes.

    • Measurements of photoinduced refractive index changes in bacteriorhodopsin films

      Ravinder Kumar Banyal B Raghavendra Prasad

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      We report the pump-probe measurements of nonlinear refractive index changes in photochromic bacteriorhodopsin films. The photoinduced absorption is caused by pump beam at 532 nm and the accompanying refractive index changes are studied using a probe beam at 633 nm. The proposed technique is based on a convenient and accurate determination of optical path difference using digital interferometry-based local fringe shift. The results are presented for the wild-type as well as genetically modified D96N variant of the bacteriorhodopsin.

    • Interferometric key readable security holograms with secrete-codes

      Raj Kumar D Mohan A K Aggarwal

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      A new method is described to create secrete-codes in the security holograms for enhancing their anti-counterfeiting characteristics. To imitate these codes is difficult as pure phase objects having complex phase distribution function are used to modulate the object beam that is recorded in conjunction with an encoded interferometric reference beam derived from a key hologram. Lloyd's folding mirror interferometer is used to convert phase variations of the reconstructed wave-front into an intensity pattern for hologram authenticity verification. Creating the secrete-codes through an interferometric reference beam from the key hologram facilitates a multi-stage authenticity verification as well as easy repositioning of the security hologram through a specific Moiré pattern generated during the verification process.

    • Effects of output coupler reflectivity on the performance of a linear cavity Brillouin/erbium fiber laser

      X S Cheng S W Harun H Ahmad

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      The effect of output coupler reflectivity (or output coupling ratio) on the performance of a linear cavity Brillouin/erbium fiber laser (BEFL) is demonstrated. The operating wavelength, output laser power and number of channels vary with changes in the coupling ratio in the linear cavity system. The optimum BEFL operation is obtained with an output coupling of 40%, i.e., 60% of the laser power is allowed to oscillate in the cavity. A stable laser comb consisting of up to 40 channels with line spacings of approximately 0.09 nm are obtained at the Brillouin pump and 980 nm pump with powers of 2.5 mW and 100 mW, respectively. The linear cavity BEFL has the potential to be used in inexpensive wavelength division multiplexing system.

    • Diffusion and transport phenomena in a collisional magnetoplasma having both streaming and temperature anisotropy: Collisional kinetics

      J N Mohanty K C Baral G Nath

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      Boltzmann-transport equation is analytically solved for two-component magnetoplasma using Chapman-Enskog analysis to include collisional diffusion transport having anisotropies in both streaming velocity and temperature components. The modified collisional integrals are analytically solved with flux integrals and perturbed kinetic equation to arrive at drift diffusion velocity and resulting transport coefficients which are markedly affected by both streaming and temperature anisotropy. The early isotropic results are recovered in the limit $V_{0} = 0$ and $T_{\|} = T_{\bot}$ which reduce to eqs (11.30) and (11.31) of [1] and eqs (2.7) and (2.13) of [2]. The electrical resistivity (n_{\bot}) diminishes sharply in fusion temperature limit $kT_{\bot} = 1$ keV. The shape of the curves for both electrical resistivity and thermal conductivity is rectangular hyperbolic. However, for low thermal ratio $(T_{\|}/T_{\bot} < 1)$, the curves are raised up and for high thermal ratio $(T_{\|}/T_{\bot} > 1)$, they are lowered down the isotropic case $(T_{\|}/T_{\bot} > 1)$, showing comparatively diminished magnitudes of the quantities.

    • Characteristics of photoconductivity in thallium monosulfide single crystals

      I M Ashraf H A Elshaikh A M Badr

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      This work elucidates the photoconductivity (PC) of thallium monosulfide single crystals. Results are obtained in the 77-300 K temperature range, 1500-4500 V lx excitation intensity, 6-18 V applied voltage, and in the 640-1500 nm wavelength range. Both the ac-photoconductivity (ac-PC) and the spectral distribution of the photocurrent are studied in different values of light intensity, applied voltage and temperature. Dependencies of carrier lifetime on light intensity, applied voltage and temperature are also investigated as a result of the ac-PC measurements. The temperature dependence of the energy gap width was described by studying the dc-photoconductivity (dc-PC).

    • Piezoelectricity in quasicrystals: A group-theoretical study

      K Rama Rao P Hemagiri Rao B S K Chaitanya

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      Group-theoretical methods have been accepted as exact and reliable tools in studying the physical properties of crystals and quasicrystalline materials. By group representation theory, the maximum number of non-vanishing and independent second- order piezoelectric coefficients required by the seven pentagonal and two icosahedral point groups - that describe the quasicrystal symmetry groups in two and three dimensions - is determined. The schemes of non-vanishing and independent second-order piezoelectric tensor components needed by the nine point groups with five-fold rotations are identified and tabulated employing a compact notation. The results of this group-theoretical study are briefly discussed.

    • Degradation process in organic thin film devices fabricated using P3HT

      Rashmi Ashok K Kapoor Upendra Kumar V R Balakrishnan P K Basu

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      The stability of regioregular poly(3-hexylthiophene 2,5-diyl) (P3HT) thin films sandwiched between indium tin oxide (ITO) and aluminium (Al) electrodes have been investigated under normal environmental conditions ($25^{\circ}$C and RH$\sim 45-50$%). Electrical and optical properties of ITO/P3HT/Al devices have been studied over a period of 30 days. Mobility 𝜇 of the order of $10^{-4}$ cm2/V-s has been obtained from the $V^{2}$ law in the as- deposited P3HT ¯lms. Scanning electron microscopy (SEM) investigations show blistering of Al contacts in devices with a poly(3,4-ethylenedioxythiophene) (PEDOT) interlayer on application of voltage whereas no blistering is seen in devices without PEDOT. The results have been explained in terms of trap generation and propagation and the moisture-absorbing nature of PEDOT.

    • Theoretical evaluation of the electron paramagnetic resonance spin Hamiltonian parameters for the impurity displacements for Fe3+ and Ru3+ in corundum

      Q Fu S Y Wu J Z Lin J S Yao

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      The impurity displacements for Fe3+ and Ru3+ in corundum (Al2O3) are theoretically studied using the perturbation formulas of the spin Hamiltonian parameters (zero-field splitting and anisotropic 𝑔 factors) for a 3d5 (with high spin $S = 5/2$) and a 4d5 (with low spin $S = 1/2$) ion in trigonal symmetry, respectively. According to the investigations, the nd5 ($n = 3$ and 4) impurity ions may not locate at the ideal Al3+ site but undergo axial displacements by about 0.132 Å and 0.170 Å for Fe3+ and Ru3+, respectively, away from the center of the ligand octahedron along the C3 axis. The calculated spin Hamiltonian parameters based on the above axial displacements show good agreement with the observed values. The validity of the results is discussed.

    • Contribution of backscattered electrons to the total electron yield produced in collisions of 8–28 keV electrons with tungsten

      R K Yadav R Shanker

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      It is shown experimentally that under energetic electron bombardment the backscattered electrons from solid targets contribute significantly ($\sim 80$%) to the observed total electron yield, even for targets of high backscattering coefficients. It is further found that for tungsten ($Z = 74$) with a backscattering coefficient of about 0.50, about $20$% of the total electron yield is contributed by the total secondary electrons for impact energies in the range of 8–28 keV. The yield of true backscattered electrons at normal incidence ($\eta_{0}$), total secondary electrons (𝛿) and the total electron yield ($\delta_{\text{tot}}) produced in collisions of 8–28 keV electrons with W have been measured and compared with predictions of available theories. The present results indicate that the constant-loss of primary electrons in the target plays a significant role in producing the secondary electrons and that it yields a better fit to the experiment compared to the power-law.

    • Energy and angular distributions of backscattered electrons from collisions of 5 keV electrons with thick Al, Ti, Ag, W and Pt targets

      R K Yadav R Shanker

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      The energy and angular distributions of backscattered electrons produced under the impact of 5 keV electrons with thick Al, Ti, Ag, W and Pt targets are measured. The energy range of backscattered electrons is considered between $E_{B} = 50$ eV and 5000 eV. The angle of incidence α and take-off angle 𝜃 are chosen to have values $\alpha = 0^{\circ}$ and 10° and $\theta = 100^{\circ}$, $110^{\circ}$ and $120^{\circ}$ respectively. The measured energy spectra are compared with the available theoretical models for $\alpha = 0^{\circ}$ and $10^{\circ}$. The elastic peak intensity of backscattered electrons is found to be a function of angle of incidence, take-off angle and atomic number of the target material. The considered theories are reasonably in good agreement with experiment for the energy spectra of the backscattered electrons having their reduced energies $\epsilon (= E_{B}/E_{0})$ in the range of 0.20 to 1.00.

    • Core temperature in super-Gaussian pumped air-clad photonic crystal fiber lasers compared with double-clad fiber lasers

      P Elahi H Nadgaran F Kalantarifard

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      In this paper we investigate the core temperature of air-clad photonic crystal fiber (PCF) lasers pumped by a super-Gaussian (SG) source of order four. The results are compared with conventional double-clad fiber (DCF) lasers pumped by the same super-Gaussian and by top-hat pump profiles.

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